Physicists have built a mirror that Snow White’s wicked stepmother would certainly not appreciate. It offers a perfect reflection.

The mirrors in our homes usually have a thin layer of metal sealed beneath a layer of glass. Light travels through the glass, reflects off of the metal and bounces back through the glass. But the mirror absorbs some light. So not all of the light completes the whole trip.

Scientists would prefer a more perfect mirror because light is crucial to many modern technol...